Color a surface according to its slope
Yeah,, you can actually do it follow this video: https://youtu.be/qSafYNQrodk?t=1330

But here you have a simplified node setup to do it:




The color ramp is used to choose the steepnes you want to select, the white parts are what is selected, so maybe you'll need to inverse the color ramp colours.
The "Average Node" of the video is actually the "Vector Math" node with the average option choosed, I don't know why he used it, the simplified node setup that I shared works equally.

How this works? I really don't know, I only know that you use the perpendicular normal axis (in this case the Z) to get the stepness in said axis and the ramp to set the threshold.

With three points (black, white, black) in the color ramp and the ease mode you can customize more the selection.